pwm: atmel: correct CDTY calculation

From the datasheet, the actual duty cycle is:

	(period - (1 / clk) * CDTY) / period

This actually correct the polarity of the PWM and solves the issue that
pwm-leds exhibits: when setting a duty cycle of 0 and then disabling a
channel, the level was wrong (1 when the polarity was normal and 0 when
the polarity was inversed).
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: NNicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-atmel.c

@@ -133,7 +133,7 @@ static int atmel_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	prd = div;
 	div *= duty_ns;
 	do_div(div, period_ns);
-	dty = div;
+	dty = prd - div;
 
 	ret = clk_enable(atmel_pwm->clk);
 	if (ret) {